A preamplifier is provided in a front-end of an optical receiver and has a function to convert a current signal to a voltage signal, the current signal being converted in a light receiving element that is a component of the optical receiver. The optical receiver is used in a system having various conditions such that the intensities of light received are different. Therefore, the preamplifier provided in the front end of the optical receiver requires wide dynamic range characteristics.
For this type of preamplifier, there has been an example of disclosure as a preamplifier having the wide dynamic range characteristics (see, for example, H. Ikeda, et al. “An Auto-Gain Control Transimpedance Amplifier with Low Noise and Wide Input Dynamic Range for 10-Gb/s Optical Communication Systems”, IEEE J. Solid-State Circuits, vol. 36, pp. 1303-1308, 2001).
Disclosed in the above literature are a circuit configuration and various characteristics of the preamplifier having the wide dynamic range characteristics.
In the preamplifier disclosed in the above literature, a feedback resistor of a negative-feedback amplifier includes a parallel circuit having mainly a fixed resistor and a field effect transistor (FET). A gate terminal voltage of the FET is changed according to light receiving power to control a conversion gain, and a wide dynamic range is realized. If the light receiving power is low, the FET is completely OFF, and the preamplifier operates so that a resistance value of the feedback resistor becomes a resistance value of the fixed resistor. Then, if the FET becomes ON with an increase in the light receiving power, the preamplifier operates so that a combined resistance value of the fixed resistor and an ON resistor of the FET becomes a resistance value of the feedback resistor, i.e., a conversion gain of the preamplifier.
It is noted that the preamplifier has a capacitor for phase compensation provided in parallel with the feedback resistor, and by optimizing a capacity value of the capacitor, non-stability of the circuit due to an increase in the light receiving power is suppressed.
However, the preamplifier disclosed in the above literature can ensure phase allowance indicating a level of stability on frequency characteristics of an open loop by the phase compensation, but, a gain peak, in turn, occurs on frequency characteristics of a closed loop. This causes the conversion gain in a certain frequency band to increase and an output voltage waveform to be distorted, called “pattern effect”.
In the preamplifier disclosed in the Non-patent literature 1, there is a time zone in which the conversion gain is large even though the light receiving power is high, during a transition state until the conversion gain becomes its stationary state. Therefore, an input terminal voltage of a load resistor becomes low in the negative-feedback amplifier in its transition state, and an excessive current flows into an input transistor and the load resistor, which may cause element destruction.
The present invention has been achieved to solve at least the conventional problems, and it is an object of the present invention to provide a preamplifier in which a waveform is less distorted and a control operation for a conversion gain is stable, without depending on whether the operation state is the transition state or the stationary state, and which has excellent wide dynamic range characteristics.